1. Field of the Invention
The present invention relates to a digital-to-analog converter, and more particularly, to a digital-to-analog converter for reducing pop noise and harmonic tone and related converting method.
2. Description of the Prior Art
In audio processing, an over-sampling 1-bit sigma-delta modulation signal is generally used for data transmission. When using this data transmission technique, however, a “pop” will be heard due to the occurrence of discontinuous points on the waveform of the audio signal (e.g., starting/finishing displaying the audio signal), causing discomfort to the listener. In addition, when finishing displaying the audio signal and under the condition that the audio signal has a lower sampling rate, if the last audio data has a higher period (i.e., more taps of a low-pass filter in a digital-to-analog converter are used) and is still in a display loop, unexpected harmonic tones are produced and a frequency of a fundamental harmonic is decreased. The frequencies of the harmonic tones are shown as follows:
            f      tone        =                            f          s                          N          tap                    ×      n        ,      n    =    1    ,  2  ,      3    ⁢    …    ⁢          ,where ftone is the frequency of the harmonic tone, fs is an over-sampling rate of the audio signal, Ntap is a number of the taps which the low-pass filter uses.
When the lowest frequency of the harmonic tones is reduced, even if the frequency of the fundamental harmonic is not within an audio band (generally defined within 20 Hz-20 kHz), the post-processing of the audio signal may be influenced when the fundamental harmonic is within 20 kHz-100 kHz.